Rust Brain Teasers/

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Puzzle 6: Explanation

Puzzle 6: Explanation

Let’s find out how floating-point approximations work in Rust.

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Test it out

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fn main() {
    if 0.1 + 0.2 == 0.3 {
        println!("Arithmetic still works.");
    } else {
        println!("Please reboot the universe.");
    }
}

Explanation

You’d expect 0.1+0.2 to equal 0.3. It does, but not in floating-point math. In this case, the floating-point approximations prevent the comparison from succeeding, as if the arithmetic no longer works.

Note: As you might have guessed by now, the floating-point comparison is fraught with errors that may not be so obvious at first glance.

Consider a long-running program that checks if it’s done by examining the floating-point product of some calculation, or perhaps by a series of unit tests that meticulously check our math functions against ...